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Let's go to Mars
c. 652 – 60 BCE

Babylon turns Mars into arithmetic

Babylonian scribes logged Mars beside barley prices for six centuries, then used a 79-year repeat to predict where it would be — with no model of why.

Two joined fragments of a clay tablet covered edge to edge in ruled lines of cuneiform, mounted against a pale museum wall.
An astronomical diary for 141 BCE. The same tablet carries the sky, the defeat of Demetrius II by the Parthians, and that year's market prices. · Image: Zunkir / British Museum, BM 45703

The oldest surviving Astronomical Diary covers 652/651 BCE, and the format barely moved for six centuries: the Moon's position night by night, the planets, eclipses, solstices and equinoxes, the level of the Euphrates, the going rate for barley, dates, mustard, sesame and wool, and what the king had been doing. Mars is there under two names — Ṣalbatānu, the workaday planet-word, and Nergal, god of plague, war and the underworld. The diaries came six months to a tablet until 61/60 BCE.

The purpose was omens; the by-product was the first quantitative sky record long enough to expose the patterns. Mars's first appearances, last appearances and stations repeat after 79 years, 36.995 synodic cycles, close enough that the calendar dates come back almost unchanged. Its passes by the reference stars repeat after 47 years, or 22.01 cycles — a period named on a tablet possibly written in the second half of the seventh century BCE.

Out of that came the Goal-Year Texts. To forecast Mars, a scribe pulled its record from 79 years earlier, applied a standing correction of about six days, and copied the result forward. No orbit, no geometry, no cause. The method held for centuries.

The limits were worst for Mars: the interval between its successive conjunctions with the Sun runs from roughly 765 to 805 days around a 780-day average, so a mean period alone forecasts badly. Mars carried two Goal-Year numbers where Saturn got by with one, and it is not clear why — modern analysis of the date corrections finds the 79-year period would have served for the star passages too. The scribes never asked why the cycles existed.

The material did not stay in Mesopotamia. Aristotle notes that the Egyptians and Babylonians had kept star observations for very many years, and that much of what the Greeks knew came from them. Hipparchus and Ptolemy worked directly from Babylonian observations and periods. Most of the tablets now sit in the British Museum, many still unpublished.